Delayed coking involves thermal decomposition of petroleum residua (resids) to produce gas, liquid streams of various boiling ranges, and coke. Delayed coking of resids from heavy and heavy sour (high sulfur) crude oils is carried out primarily as a means of disposing of these low value resids by converting part of the resids to more valuable liquid and gaseous products, and leaving a solid coke product residue. Although the resulting coke product is generally thought of as a low value by-product, it may have some value, depending on its grade, as a fuel (fuel grade coke), electrodes for aluminum manufacture (anode grade coke), etc.
The feedstock in a delayed coking process is rapidly heated in a fired heater or tubular furnace. The heated feedstock is then passed to a large steel vessel, commonly known as a coking drum that is maintained at conditions under which coking occurs, generally at temperatures above about 400° C. under super-atmospheric pressures. The heated residuum feed in the coker drum results in volatile components that are removed overhead and passed to a fractionator, leaving coke behind. When the coker drum is full of coke, the heated feed is switched to a “sister” drum and hydrocarbon vapors are purged from the drum with steam. The drum is then quenched first by flowing steam and then by filling it with water to lower the temperature to less than about 300° F. (148.89° C.) after which the water is drained. The draining is usually done back through the inlet line. When the cooling and draining steps are complete, the drum is opened and the coke is removed after drilling and/or cutting using high velocity water jets.
Cutting is typically accomplished by boring a hole through the center of the coke bed using water jet nozzles located on a boring tool. Nozzles oriented horizontally on the head of a cutting tool then cut the coke so it can be removed from the drum. The coke cutting and removal steps add considerably to the throughput time of the overall process. Thus, it would be desirable to be able to produce a coke that can be removed from a coker drum with little or no cutting. Such coke would preferably be a substantially free-flowing coke. It would also be desirable to be able to safely remove such substantially free-flowing coke at a controlled flow rate.
Even when the coker drum appears to be completely cooled, some areas of the drum may still be hot. This phenomenon, sometimes referred to as “hot drum”, may be the result of a combination of different coke morphologies being present in the drum at the same time. For example, there may be a combination of one or more needle coke, sponge coke or shot coke. Since unagglomerated shot coke may cool faster than other coke morphologies, such as large shot coke masses and sponge coke, it would be desirable to produce predominantly substantially free-flowing unagglomerated shot coke in a delayed coker, in order to avoid or minimize hot drums.